1. Technical Field of the Invention
The present invention relates to cellular telephone networks and,, in particular, to the process for effectuating mobile assisted handoff (MAHO) within a cellular telephone network.
2. Description of Related Art
Reference is now made to FIG. 1 wherein there is shown a schematic view of a cellular telephone system 10 including a plurality of individual cells 12. Each cell 12 operates with a different assigned set of channels. In non-multiplexed communications systems, any given frequency comprises a channel. In multiplexed communications systems, however, each frequency is divided into a plurality of time slots which comprise the channels. The set of channels assigned to each cell 12 includes at least one control channel (CCH) including a reference, pilot and/or measurement channel operable in either or both an analog and/or a digital mode, and a plurality of traffic (or voice) channels (TCHs) also operable in either or both an analog and/or a digital mode. Sets of assigned channels are different for adjacent cells 12, and such sets are not repeated except for cells that are far enough away from each other to minimize the likelihood of adjacent channel or co-channel interference.
A base station 14 including a plurality of transceivers (not shown) capable of independently operating on each of the assigned set of channels is provided for each of the cells (one set of channels per base station). Via the transceivers, the base stations 14 engage in communications with plural mobile stations 16 operating within the area of the associated cell 12. The control channels in the set of channels assigned to each cell 12 are used to carry system control signals between the base station 14 and proximately located mobile stations 16. Such control signals include page signals, page response signals, location registration signals, voice channel assignments, maintenance instructions, and cell selection or reselection instructions. The plurality of traffic channels in the set of channels assigned to each cell 12 are used to carry subscriber voice or data communications as well as system handoff communications between the base station 14 and proximately located mobile stations 16.
The base stations 14 further communicate via signaling links and voice trunks 22 with a central control station, commonly referred to as a mobile switching center 18, which functions to control operation of a portion of the system 10. In particular, the mobile switching center 18 operates to selectively connect subscriber voice and data communications to the mobile stations 16 through the base stations 14. Thus, the mobile switching center 18 controls system operation through and in response to the transmission of control signals over the control channels to set-up on the traffic channels calls that are either originated by or terminated at the mobile stations 16. The mobile switching center 18 further controls, through and in response to control and traffic channel transmissions, as well as measurements made on control and traffic channels, the handoff of a subscriber communication from a traffic channel of one cell 12 to a traffic channel of another cell as the subscriber mobile station 16 moves throughout the cellular service area during an ongoing communication.
A brief description of the handoff process is now provided. An ongoing call communication is being carried by one selected traffic channel. As the mobile station 16 moves around the broadcast coverage area of a currently serving cell 12, the mobile station makes signal strength measurements on both the currently used traffic channel and a reference one of the included control channels (also referred to as a pilot channel or measurement channel) for each of the neighboring cells identified on a neighbor list supplied to the mobile station by the mobile switching center. At the same time, the base station 14 for the serving cell 12 may make uplink signal strength measurements on the currently used traffic channel. When the mobile station 16 moves towards the edge of the currently serving cell 12, system evaluation of the downlink and uplink measured signal strengths for the currently used traffic channel and downlink measured signal strengths for the control (reference) channels of the neighboring cells provides an indication that a handoff is necessary. At that point, the system identifies potential candidate cells 12 for handoff and requests that the base stations 14 for these cells make and report on measured uplink signal strength with respect to the current traffic channel. By processing all of the made uplink and downlink signal strength measurements, the system chooses a new cell 12 (and associated base station 14) to handle continuation of the call. A traffic channel from the set of channels assigned to the chosen new cell 12 is then selected, and a command is transmitted from the mobile switching center 18 over the current traffic channel ordering the mobile station 16 to switch communications to the selected traffic channel in the new cell. At the same time, the mobile switching center 18 re-routes the call from the base station 14 in the old cell 12 to the base station 14 for the new cell to complete the handoff process.
In some cellular telephone systems, the traffic channels and the control channels utilize different types of signal modulation schemes. For example, in a digital cellular telephone system such as that proposed by the digital advanced mobile phone service (D-AMPS), the traffic channels utilize quadrature phase shift keying (QPSK), and at least the measurement (reference or pilot) ones of the control channels utilize frequency shift keying (FSK) as their respective modulation schemes. It has been noted, however, that in some cases significant discrepancies in MAHO made downlink measured signal strength may exist between the traffic channel of the serving cell and the control channel of a neighboring cell. These discrepancies have been linked, at least in part, to the differences in modulation types used on the signals. Additionally, with specific reference to cellular systems supporting plural hyperband communications (for example, operation at both 800 MHz and 1900 MHz), it has been noted that significant discrepancies in MAHO made downlink measured signal strength may exist between the traffic channel of the serving cell operating in one hyperband and the control channel of a neighboring cell operating in another hyperband.
Reference is now made to FIG. 2 wherein there is shown a graph illustrating a comparison of downlink signal strength measurements made by a mobile station on a reference (control) channel versus a traffic channel. The graph of FIG. 2 is generated from an experiment recording downlink signal strength measurements on the reference (control) channel of the serving cell and the traffic channel of the currently serving cell for all mobile station handoff requests made over a twenty-four hour period. In the experiment, the reference (control) channel utilized frequency shift keyed (FSK) modulation, and the traffic channel utilized quadrature phase shift keyed (QPSK) modulation, and both channels were in the same hyperband. The recorded measurements for each handoff were compared (i.e., control channel signal strength for the serving cell minus traffic channel signal strength for the serving cell), with the comparison results across the twenty-four hour test period used to generate a probability density function (pdf). The peak of the probability density function illustrates a large number of handoff instances wherein the mobile station downlink traffic channel signal strength measurements with respect to the serving cell are between four and five decibels (4-5 dB) less than the mobile station downlink reference (control) channel signal strength measurements with respect to the target cell. Similar results have been noted when the reference (control) channel of the serving cell and the traffic channel of the currently serving cell utilize different hyperbands.
It has further been noted that such modulation and/or hyperband discrepancies in MAHO made downlink measured signal strength may adversely impact cellular telephone system performance. For example, the discrepancies, when not accounted for during system handoff processing, may causing oscillating (i.e., ping-pong) handoffs as the mobile station moves along the boundary between two adjacent cells. There is a need then for a mechanism that would allow the cellular telephone system to account for modulation scheme and hyperband influenced discrepancies in MAHO made downlink signal strength measurements.
Reference is now made to FIG. 3 wherein there is shown a graph illustrating a comparison of downlink signal strength measurements made by a mobile station on one traffic channel versus another traffic channel. The graph of FIG. 3 is generated from an experiment recording downlink signal strength measurements on a first traffic channel of the serving cell and a second traffic channel of the currently serving cell in relation to mobile station handoff requests made over a twenty-four hour period. In the experiment, each of the first and second traffic channels utilized quadrature phase shift keyed (QPSK) modulation, and each were within the same hyperband. The recorded measurements for each handoff were compared (i.e., first traffic channel signal strength for the serving cell minus second traffic channel signal strength for the serving cell), with the comparison results across the twenty-four hour test period used to generate a probability density function (pdf). The peak of the probability density function illustrates a large number of handoff instances wherein the mobile station downlink first traffic channel signal strength measurements with respect to the serving cell and second traffic channel signal strength measurements with respect to the serving cell are nearly the same. This shows that downlink signal strength measurements for traffic channels using the same QPSK modulation scheme are comparable. Similar results occur with respect to evaluating FSK modulation when comparing downlink signal strength measurements on a first control channel of the serving cell and a second control channel of the currently serving cell.